Star Trek is to Prime Directive as Lord of the Rings is to D&D. Prime Directive is heavily influenced by Star Trek, to the point of having a Star Fleet and Vulcans (and obviously a Prime Directive), but the continuity and much of the universe is significantly different. PD is based on Star Fleet Battles which was originally a Star Trek wargame, but in later editions SFB was split off and re-written and became it's own beast.

I don't have SFB or PD, I just read the FAQ. _________________"Any sufficiently analyzed magic is indistinguishable from science!" - Agatha Heterodyne